1. Field of the Invention
2. Description of the Related Art
Paved surfaces are required in many situations where the impermeability of a conventional pavement is essential, but not without disadvantages. In some situations--refuelling stations, chemical loading bays and vehicle parks, for instance--there may be risk of spillage of fluids which must be prevented from entering the drainage system and so passing by uncontrolled discharge into the environment, and must be prevented from entering the environment at all without preliminary treatment.
Where there are large areas of pavement--motorways, for instance--flash storms may cause such flooding that existing storm-water provisions become overloaded. There have been occasions, too, when emergency treatment of spillage after accidents involving chemical tankers has resulted in serious damage to the environment over side areas.
Where risks of chemical contamination are low, various forms of porous pavement have been employed, allowing spillage to soak away into the ground beneath. There is, however, a limited rate at which ground can absorb moisture, and there remains a risk of flooding when rainfall is very high.
WO-A-8 500 631 describes a structure for controlling an artificial water table beneath a tennis court or agricultural land in which a ballast layer of generally uniform size rock is laid on top of a watertight membrane in a basing, the ballast layer being extremely permeable by water. The ballast layer is covered by a permeable membrane which is covered by a layer of finely divided particles which are prevented from penetrating the ballast layer by the permeable membrane. Water diffuses through the fine layer by capillary action. Water is inserted in the ballast layer, causing the water table in it to rise to and above the permeable membrane. The amount of water provided to a finished layer above the fine layer can be adjusted by raising and lowering the water level.
DE-A-3 041 624 describes a method of sealing and draining an area of ground where the regions to be sealed are limited by kerb stones, some of which have discharge channels.